Scientists utilize tools known as frequency comb lasers to detect methane in the atmosphere above oil and gas sites and to identify potential infections through human breath samples. A recent study aims to enhance the accuracy of these sensors.
What’s the secret to improving a quantum sensor? Just apply a little pressure.
For the first time, researchers have successfully implemented a method known as “quantum squeezing” to boost the gas detection abilities of optical frequency comb lasers. These highly accurate sensors function much like fingerprint scanners for gas molecules. They’ve been used to identify methane leaks in the air over oil and gas operations, as well as to detect indicators of COVID-19 in human breath samples.
In a series of laboratory tests, scientists have outlined a strategy to make these measurements even more responsive and efficient—effectively doubling the speed of frequency comb detectors. This research is a joint effort by Scott Diddams from CU Boulder and Jérôme Genest from Université Laval in Canada.
“Imagine a scenario where you need to detect tiny amounts of a harmful gas leak in a factory situation,” explained Diddams, a professor in the Department of Electrical, Computer and Energy Engineering. “Reducing the detection time from 20 minutes to just 10 could significantly enhance safety.”
He and his team shared their results on January 16 in the journal Science, with postdoctoral researcher Daniel Herman leading the effort.
Unlike standard lasers that emit light in only one wavelength, frequency comb lasers release pulses across thousands to millions of wavelengths simultaneously. In this study, the researchers manipulated the light pulses using regular optical fibers, allowing them to “squeeze” the light and enhance certain properties while slightly randomizing others.
This advancement demonstrates a breakthrough against the natural unpredictability and variations present at very small scales in the universe.
“Overcoming quantum uncertainty is challenging and doesn’t come easy,” stated Diddams. “However, this is a significant advancement towards creating a powerful new class of quantum sensors.”
Manipulating Photons
The findings mark another step forward in the development of frequency comb technology, which was pioneered at JILA, a collaborative research center between CU Boulder and the National Institute of Standards and Technology (NIST). Diddams was part of the original team led by Jan Hall at JILA that developed frequency combs in the late 1990s, with Hall later being awarded the Nobel Prize in Physics for this innovation in 2005.
When these laser pulses travel through the atmosphere, various molecules absorb specific wavelengths of light, causing some colors to disappear altogether. By analyzing the missing colors, scientists can determine which gases are present in the air, similar to a hair comb that has lost some teeth, which is how the technology got its name.
However, these measurements come with built-in uncertainties, according to Diddams.
“Light consists of tiny particles called photons, and while lasers may seem orderly, the individual photons behave randomly,” he explained.
“When detecting these photons, they don’t arrive at a consistent rate; instead, they come at random intervals,” Diddams noted. This randomness introduces a level of “fuzziness” in the data acquired from a frequency comb sensor.
Here’s where quantum squeezing comes into play.
Applying Squeeze
In quantum physics, there are many interrelated properties, meaning that getting a precise measurement of one often leads to less precise results for another. A classic example is the measurement of an electron’s speed and position—you can know either one accurately, but never both at once. Squeezing is a technique that optimizes one measurement type while compromising another.
Throughout various lab experiments, Diddams and his team accomplished this in a straightforward manner: they directed their frequency comb laser pulses through a standard optical fiber, not too different from what carries internet service to your home.
The structure of the fiber modified the light in a way that allowed photons from the lasers to arrive at a more consistent interval. However, this increased regularity made it slightly more challenging to measure the light’s frequency or the oscillation of the photons creating specific colors.
Nevertheless, this compromise enabled the researchers to detect gas molecules with significantly fewer errors than before.
In the lab, they tested their method using hydrogen sulfide, a compound commonly found in volcanic eruptions with a smell likened to rotten eggs. They found they could identify these molecules roughly twice as fast with their squeezed frequency comb compared to conventional methods, and were able to achieve this improved detection across a much broader range of infrared light—about 1,000 times more than previous efforts.
The team acknowledges that further work is needed before their innovative sensor is ready for practical implementation.
“Our results indicate we are closer than ever to deploying quantum frequency combs in real-world applications,” Herman stated.
Diddams concurred, saying, “Researchers refer to this phenomenon as a ‘quantum speedup.’ We’ve managed to manipulate the foundational uncertainty relations in quantum mechanics to measure things more efficiently.”
Additional CU Boulder contributors to this study include Professor Joshua Combes, graduate students Molly Kate Kreider, Noah Lordi, Eugene Tsao, and Matthew Heyrich, along with postdoctoral researcher Alexander Lind. Mathieu Walsh, a graduate student from Université Laval, also contributed as a co-author.
The project at CU Boulder received funding from the U.S. National Science Foundation through the Quantum Systems through Entangled Science and Engineering (Q-SEnSE) Quantum Leap Challenge Institute and from the Office of Naval Research.
